Objectives To evaluate the relationships between social behavior, reproduction, and ecology in wild muriqui monkeys by long-term observational and non-invasive methods. Since 1982 we have studied the behavior and ecology of one group of muriquis inhabiting an 800 hectare forest in Minas Gerais, Brazil. This is the only ongoing long-term field study of this highly endangered primate. Field observations have described the muriqui's unusual egalitarian society and pattern of female-biased dispersal and male philopatry while documenting the effects of seasonal changes in food availability on diet. Demographic records of recognized individuals have yielded the only available data on basic life history variables, including age at first reproduction and interbirth intervals for this species. Correlations between reproductive events and ecological variables indicate pronounced reproductive seasonality, suggesting that components of muriqui diets may regulate their fertility. Dietary components may also be linked to the regulation of intestinal parasite infections. Collaborations with Brazilian scientists and researchers at UNC-Asheville and Cornell University are identifying muriqui parasites and analyzing their plant foods for anthelminthic and phytosteroid activity. Long-term demographic and ecological data are also providing the basis for population viability analyses that will be critical to the development of conservation plans for this primate. Key words reproduction, parasites, fertility